ARRL : 8th Computer Networking Conference - October 7, 1989 - page 203-209
DAMA - A NEW METHOD OF HANDLING PACKETS ?
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by Detlef J. SCHMIDT, DK4EG Steinbrecherstr. 22 D-38106 BRAUNSCHWEIG
NORD> User :
-------------------------------
There is no difference between regular CSMA and DAMA in this case. Because
it is always up to the master (node) to act first, it could send one or
more I-frames or a poll to the user. The user will acknowledge I-frames
immediately with an RR#, but could also send its own I-frames with the
corresponding count (having to correct the count on the sent I-frame serves
the same purpose as an ACK with AX.25). The meaning of the Poll/Final bit
remains unchanged.
Data transfer : User --> Node :
-------------------------------
As mentioned before, the node will send polls to all users that are
uplinked to it and the user will not respond until it receives this poll or
an I-frame from the node. It may be wise to point out that when a user is
polled he must always come back with some kind of response, even if it is
an RNR#. If the node fails to hear any kind of response from the user then
it assumes something went wrong (such as a collision) and moves on to the
next user on its polling list.
This method of always waiting for a poll before transmitting is the central
aspect used to avoid collisions in a situation where hidden stations exist.
This is in contrast to the usual CSMA method where several stations can
actually transmit at the same time. Additionally the problem of deadtime
collisions is resolved. Deadtime refers to the period from when the TNC
realizes the channel is free and starts transmitting, to when he has been
on the air long enough for other TNCs to recognize his carrier. This is
really not a rare case, as exemplified by the case where two or more TNCs
are waiting for a digipeaters carrier to vanish so that they can leap on
the frequency. Using DAMA the node will not acknowledge received frames the
instant it hears them. Instead it will first service all other uplinked
stations and then come back with an RR# to the sending stations I-frames
along with a poll to that station. This poll basically says "Have you got
any else for me ?".
Disconnecting :
---------------
If the master intends to cut the connection, it will send the usual
DISC-frame to the user. The user will then promptly respond with the
UA-frame (final bit set). If the node fails to receive the UA and again
sends a DISC-frame, the user will respond with a DM-frame. This is
identical to the actual CSMA version.
When the user wants to disconnect from the node, he will wait to send his
DISC-frame until polled by the master. AT this point it makes no major
difference whether the node responds to the user right away with a UA or
goes through another polling cycle to do so, however an immediate UA is
preferred.
UI-frames :
-----------
In CSMA as well as in a DAMA environment, the UI-frames are treated in a
special way. I.E. These frames are used to carry some information besides
the regular protocol traffic. Normally UI-frames are never sent from a user
to a node, and it is not good headwork to make a habit of making UI-frame
direct QSOs on the input frequency of a node. However, in contrast to a
duplex system, it is possible to actually do this. So although the rare
UI-frames will reduce the throughput to the CSMA value, it will not drop
the much lower ALOHA value that would occur with a duplex digi having a QSO
on its input frequency. UI-frames originated by the node are no problem
since all stations receive these frames.
Other protocol elements :
-------------------------
So we have gone from the beginning to the end in describing a complete DAMA
session. We have not translated each and every AX.25 element into one that
has a special significance to DAMA. This is not required since many of them
will keep their initial meaning. DM, RNR, REJ, etc will all be used as they
were before. The only deviation from the pure CSMA version is in the fact
that the users will only be allowed to transmit these frames after
receiving permission from the master (node) in the form of a poll. The node
will only transmit these frames after all other users on its list are
served by completion of one polling cycle.
Compatibility of DAMA and CSMA :
--------------------------------
One advantage of the DAMA method is that it does not require everybody to
change everything all at once. However as additional users convert their
TNCs to work with DAMA the more pronounced will become the increase of
throughput. Even stations that are waiting to switch over could help to
increase the areas throughput by changing a few operational parameters. For
example the delay between the reception of a frame and the TNCs response
(sometimes called T2 or DWAIT) should be reduced to a value under 1 second.
In addition the time interval from when an I-frame is sent to when the TNC
sends an RR# to ask for a pending ACK, should be set to a value that is
clearly higher than the time between two polls of the master (usually more
than 30 seconds at 1200 Bd).
To fully benefit from DAMA both the node and the user must work together in
the master/slave relationship. Assuming that the users TNC is capable of
both the normal and the DAMA mode, there still remains the problem of how
to tell the user to "turn DAMA mode on". There are several ways that this
could be done:
1. Automatic detection of the protocol version by means of the protocol
identifier byte or reserved SSID-octet-bits of the node (preferred
version).
2. Implementation of a channel specific parameter which controls the
protocol version.
3. Implementation of a new UPLINK command besides the current CONNECT
command.
4. Implement a further protocol element such as a SABM-frame (similar to
X.25) so that at connect time the node could alert the user to the
increased features.
In case #1 of the above it would be sufficient to tell the user to switch
DAMA mode only once, at connect time. This state would then remain in
effect until disconnect. However since there is no PID field in
SABM-frames this information has to be carried in some other way, such as
utilizing the dormant bit 5 of the master's SSID address field. It is
proposed that DAMA test versions set this bit to 0 to convey the necessary
information to the users TNC.
Conclusion :
------------
The existing AX.25 version was established in 1982 when packet radio was
not widespread as it is today. Most stations in the beginning were pretty
much equal and there was no distinction made between DTE and DCE functions.
However with the implementation of wide area networks not all stations are
performing the same function. In fact today the network nodes are acting in
DCE function considering their control and information exchanging aspects.
These functions will be better served with the implementation of DAMA.
The methods discussed in this article could increase the throughput on an
AX.25 channel tremendously. One advantage is the avoidance of system
breakdown which occurs with channel overload. Using DAMA, the throughput
will increase continuously up to its maximum. There is no foldback effect
like that which occurs using CSMA where at a special limit (above ca 60%)
the throughput is actually reduced.
There is also a strong "social" aspect of DAMA wherein even the weak
stations can work through the node reliably without being overpowered by
stations close to the node.
It is possible to make direct connections with other HAMs on the uplink
frequency unlike that of a duplex system. In addition the users TNCs still
retain the digipeater capability inherent in our present simplex system.
All protocol elements keep their original meaning which allows both
versions to be utilized on the same frequency, yet throughput increases as
more and more users switch over to the new method.
Literatur
Fox,T. AX.25 Level 2 protocol specifications AMRAD
Kauffels,F.J. Lokale Netze R.Mller Verlag
Mahle,C. Satellite Scenarios and Technology IEEE J. on selected
Hyde,G for the 1990's Areas in communication
Inukai,Th. May '87
Schmidt,D.J. DAMA, ein neues Verfahren fr cq-DL, 4/89
Packed-Radio?
Schmidt,D.J. Synchrone DF-Protokolle mit 6809-Micro- TU-Script BS '81
Computern in heterogenen Sternnetzen
Tanenbaum,A. Computer Networks Prentice Hall Verlag
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DAMA was implemented in 1989 for the first time with TheNetNode
(ST and PC, DAMA-Master) and TheFirmware (TNC2, DAMA-Slave)
software by the NORD>